What physical quantity is the same for X-rays of wavelength \( 10^{-10}\,\mathrm{m} \), red light of wavelength 680 nm and radiowaves of wavelength 50 m?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Speed in vacuum
X-rays, red light and radio waves are all electromagnetic waves; they differ only in wavelength. Every electromagnetic wave travels in vacuum with the same speed
\( \displaystyle c = \frac{1}{\sqrt{\mu_0 \varepsilon_0}} = 3\times10^{8}\,\mathrm{m\,s^{-1}} \)
NCERT (2026-27):
Points to Ponder – “1. The basic difference between various types of electromagnetic waves lies in their wavelengths or frequencies since all of them travel through vacuum with the same speed.”